1. Field of the Invention
This invention relates to an improved method for making self-aligned implants in planar, epitaxial base, bipolar transistors. More particularly, the invention relates to a method for making self-aligned extrinsic base and pedestal collector implants in a planar epitaxial-base bipolar transistor or a heterojunction bipolar transistor.
2. Description of the Prior Art
As will be appreciated by those skilled in the art, in a bipolar transistor, an increase in collector doping in the region directly below the emitter can improve the high frequency performance of the transistor.
In a planar epitaxial-base bipolar transistor, the intrinsic and extrinsic bases are formed in a common layer, and the emitter window represents a region where the emitter polysilicon comes in contact with this base layer. Since it is desirable to have a very thin base layer, the base resistance may be excessive unless the conductivity of the extrinsic base is raised. Although the emitter electrode may be used to mask an extrinsic base implant, there is no convenient way to self-align this extrinsic base implant with the smaller emitter window, or to self-align the emitter window and the extrinsic base with a pedestal collector implant.
A heterojunction bipolar transistor (HBT) resembles an epi-base transistor in that the base is typically a blanket layer, but the emitter layer (which has a wider bandgap than the base) is typically deposited on top of the base in a common epitaxial deposition. Contact to the base layer is usually made by etching through the emitter. Alternatively, in order to form a planar device, an implant can be used to convert a region of the emitter into a thicker extrinsic base. Again, a method for self-aligning this implant to a small emitter window and a pedestal collector implant would be advantageous.